1. Field of the Invention
The present invention relates to a load driving circuit.
2. Description of the Related Art
In a switching power supply circuit or inverter circuit, input voltage is converted by switching a power MOSFET, in general. In the switching of power MOSFET, a load driving circuit with high current driving capacity is employed in many cases. FIG. 3 is a diagram illustrating a general configuration of a load driving circuit for driving the power MOSFET (Japanese Patent Laid-Open Publication No. 2007-142816, for example). A load driving circuit 100 is a circuit for changing according to an input voltage IN an output voltage OUT of a class AB output circuit 230 with high current driving capacity. The load driving circuit 100 includes an NPN transistor Q100, a PNP transistor Q110, current sources 200 and 210, a switch 220, and the class AB output circuit 230. The class AB output circuit 230 includes NPN transistors Q120 and Q130, PNP transistors Q140 and Q150, and diodes 300 and 310. It is assumed that the load driving circuit 100 is an integrated circuit.
If a high-level (hereinafter referred to as H level) input voltage IN is applied to a base electrode of the NPN transistor Q100, which is an input of the load driving circuit 100, the NPN transistor Q100 and the current source 200 constitute an inverter, so that the NPN transistor Q120 is turned off. Therefore, the PNP transistor Q150 is turned off. Since the switch 220 is turned on in response to the input signal IN at the H level, a current I1 of the current source 210 flows through the PNP transistor Q110. Since the PNP transistor Q110 and the PNP transistor Q140 constitute a current mirror circuit, the PNP transistor Q140 supplies a current corresponding to the current I1 to the NPN transistor Q130. As a result, since the NPN transistor Q130 is turned on, the output voltage OUT becomes the H level. On the other hand, when the input voltage IN becomes a low level (hereinafter referred to as L level), the NPN transistor Q120 is turned on. Since the switch 220 is turned off, the PNP transistor Q110 is turned off, and the PNP transistor Q140 is also turned off. Therefore, the output voltage OUT reaches the L level. The diodes 300 and 310 are elements for lowering the voltage of the base electrode of the NPN transistor Q130 so that the NPN transistor Q130 is turned off earlier if the NPN transistor Q120 is turned on.
As mentioned above, in the load driving circuit 100, it is required that the NPN transistor Q130 be turned on in order that the output voltage OUT shall be at the H level in accordance with the input voltage IN. The NPN transistor Q130 is designed greater in size so as to heighten the current driving capacity of the class AB output circuit 230. As a result, a parasitic capacity of the NPN transistor Q130 is increased. Thus, a time from when the input voltage IN has reached the H level until when the output voltage OUT reaches the H level in a case where the current I1 of the current source 210 is small, is longer than the time in a case where the current I1 is great, as shown in FIG. 4. Therefore, the current I1 of the current source 210 needs to be increased in order to reduce a rise time of the output voltage OUT, and thus, there is a problem that consumption current of the load driving circuit 100 is increased.